


When Hearts Collide

by skerb



Category: Undertale (Video Game)
Genre: ATTL-typical headcanons, Accidents, Bodyswap, Chronic Pain, M/M, Mental Health Issues, Minor Injuries, POV Alternating, Sans/Underfell Sans (Undertale), Swearing, Underfell Sans (Undertale), background fellcest, background kedgeup, kustard - Freeform
Language: English
Status: In-Progress
Published: 2021-02-05
Updated: 2021-02-25
Packaged: 2021-03-16 22:22:50
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 4
Words: 9,710
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/28963860
Author URL: https://archiveofourown.org/users/skerb/pseuds/skerb
Summary: It was a fluke -- just toss something into the rift and get it to close again, but the machine had other ideas. What starts off as an innocent enough task slings Sans and Red into each other's bodies after the machine explodes. In the meantime, they have to keep this on the down low, while also working through just how each other -- and those around them -- tick.AnATTL-inspiredAU.
Relationships: Minor or Background Relationship(s), Sans/Sans (Undertale)
Comments: 80
Kudos: 161





	1. Chapter 1

**Notes for the Chapter:**

> Red's POV

There’s a jarring sensation when the dust settles, thick in the air like fine powder. Red finds himself frozen on the ground, having last remembered throwing himself between Sans and the blast.

For a soul-shuddering moment, he thinks he failed, but something isn’t quite right. It’s different from the off, iron-like scent of monster dust. It smells of fibreglass, ozone, and ceiling plaster. It takes the span of several moments for Red to register that Sans probably isn’t dead, and he’s on the floor next to him instead of pinned down under his body.

But no, there’s someone above him, crowding his personal space like a mugger in a back alley. It’s not Sans -- they smell like the faint whisper of weed, smoke, and the tang of leather. Red’s defenses hike up and soar over a thousand, though his magic’s either out of reach or somehow in the commotion he fried himself.

If his vision wasn’t so clogged with the remains of the basement, Red would have a better view. He doesn’t give it a second thought. He kicks up and punts the body in the chest so they stop looming over him.

He hears Sans groan. Then swear. Then the body above him rocks over and collapses next to him. Oops. Red draws in haggard breaths, noting the pinched pain behind his ribs like a fresh slap. Goddamn emotions. Either that, or the blast somehow pierced through his ribs and thrashed at his soul for good measure. It feels as hot as a welt.

“Stay out of the blast zone next time, asshole,” Red says, dragging an arm over his eyes to shield them from the rain of pebbly debris.

Only it’s not Red’s usual, gravelly voice, worked rough from yelling and about 30 years of binge drinking and smoking. He freezes for another moment, taking stock of where he is and where Sans might be under the cloud of plaster dust.

It’s Sans’s voice.

The grin Red usually dons to cover up his uneasiness suddenly drops. An icy shot goes through him when he hears a cough, rough and caked with dust like he’s inhaled spice. Red rolls over in a flash, throwing his body over the interloper to pin them down.

His jacket. His clothes. Red stares down at their dazed expression -- _his face._ Red recognises the blue under his drywall-dusted sleeves and freezes. Sees the smooth, unscarred fingers of his hands. He’s aware of explanations and cautionary tales of physics-bending machines gone wrong, but all Red can think is one thing.

_What the fuck?_

He doesn’t move. He waits for his body to snap the fuck to it and process whatever the hell happened. As predictably as what can be expected of his alter, Red watches as _his_ face goes through an alarming and irritating span of confusion, perplexed idiocy, bewilderment, and rising panic.

“Stop making those fuckin’ faces,” he says with Sans’s voice. It makes Red pause, though he’s not sure how he knows that Sans is in his body just as he’s in _his._ Probably the lack of LV that pings back at him and the fact that the judge is a silent observer in his head as it always is despite his body very readily murdering people in the past.

Sans’s eye lights are white, not tainted red like his own. It’s disarming to see his face with that contrast when it’s been gone ever since he could remember.

Finally, like he’s croaking out some form of protest, Sans sputters, “What the fuck happened?”

Red doesn’t let him go. He feels Sans test the weight above him like he’s done so many times to Sans before. Red glances over his -- Sans’s -- shoulder instead, to the smoking wreck of the machine that had gone haywire over the past couple of hours. They’d tried to stop it from fizzling out, to make sure that they’d only need to plug it in when it came time to address their guilt of leaving starving monsters back in Red’s universe.

And welp, here’s where they landed. This is why you never go soft. Shit always hits the fan. Never fails.

Sans swallows after another cough. “Red… that’s _you_ there, right?”

It’s probably messing him up as much as Red is to be in an entirely different place, to see his body glaring down at him. Red thinks about his LV-tainted eye lights and looks back to him. Sans tenses in his body, apparently content to play sitting duck.

“Motherfucker,” Sans breathes, all relieved as his grin tentatively ticks up.

“Why?” Red spits. “Why are you sounding like that? Fuckin’ stop it. We’re _fucked.”_

As if by his command, the grin slips away. Red hates how his body looks like that. He glowers down, wanting to dig into his own skull and pry open the leys so he never makes such stupid faces again.

“Easy,” Sans says, taking measured breaths. “We’ll sort through it.”

It’s so weird for Sans to be the one to comfort him. Red scoffs, realising how infuriating this all is. Sans doesn’t normally have such snotty behaviour to the point where Red feels extremely aggravated, and Red doesn’t sound _soft._ He just _doesn’t._

Mistaken identities aside, Red can’t control this. It’s already happened. The lab smells like burned oxides and the structural integrity is probably shit. He doesn’t want to look deep inside Sans’s body and find that he’s a decimal point away from kicking it, but oddly enough he feels fine. Better than how his own body looks.

He glowers down at Sans, sizing him up. Red notes the hairline crack pierced down over his false tooth, his scars, and the deep set crimson grooves honed into the bone under his eyes. In his body, Sans glares back at him, though his natural standoffishness curdles like sour milk. There’s just no heat behind it.

Red snorts despite himself. Then he gets up off of his body -- _Sans_ \-- and realises with a pang of agony that his soul is acting up. It twinges hard enough to make him grimace, but he sucks in a shallow breath to soothe the ache. He doesn’t do the thing where Sans rubs over his chest because that’s such a fucking tell. Sans gives him an odd look anyway.

Growling, Red leans forward and grabs a hold of his jacket, tugging his body forward for good measure. “I said _stop_ makin’ those stupid faces with my face!! You’re makin’ me look like a grade-A idiot!!”

Sans puts up his hands in a placating gesture. He apparently has no complaints about how Red’s behaving in his own body, though he doesn’t seem to have as many hang-ups as Red does about being an asshole.

“Ok, ok. Lemme get the resting bitch face down pat.” Sans glowers at him. It’s hollow, but Red distractedly rubs at his neck, feeling prickles stick up on his bones. He feels the collar’s absence like a slap in the face. “Better, mister sourpuss? Fuck. Let’s get our bearings before you have a total fucking meltdown over this.”

Red takes a breath as though to argue, but even that hurts a little bit. Jeez, does Sans always feel this way? Taking a moment to just ease back and breathe, Red mutters some choice swears under his breath. He’s got a feeling it’s because Sans has been downplaying how bad his soul cracks are lately. Attempting some chivalric bullshit to save Edge the effort of healing him.

Edge.

“Fuck.” Red lets himself fall back onto the floor, even as his new body complains about that.

“What?” Sans doesn’t hesitate to demand. Red reaches up to push him away when Sans gravitates towards him. “Fucking chill, man. That’s my body you’re in.”

“Weirdest letter to Penthouse I’ve ever heard,” Red mutters wryly to himself. “God, no. We knew the risks. But fuck.”

“But fuck is never a great time to begin with.” Red makes significant eyes at Sans, and oh shit, he wasn’t expecting Sans’s magic to seep through whatever cognisant connection they’ve got going on. Seeing the flush so different from his own on his body makes Red shudder on the side of Uncanny Valley. “Ok, shut up.”

“The boss is gonna have a fucking field day with me,” Red groans, wiping over his -- _Sans’s_ \-- face. He catches the odd look Sans gives him, something lost between righteous _I-told-you-so_ and hopeful awakening libido party. “I knew this was a bad idea.”

“Actually, the bad idea was continuing to work on the machine _while_ it was malfunctioning,” Sans corrects, somehow scoring Red’s blasé tone tit for tat. “You know, when the smoke started pouring out?”

Red growls in frustration. “Ok, but leaving it to open a fuckin’ wormhole to the nth dimension wasn’t an option. You’re fucking welcome, by the way, mister ‘not killed by a flying spring’.”

Sans snaps his mouth shut. He seems to have trouble with that. Come to think of it, Red hasn’t come this close to biting off his own tongue in a long time. He just tests Sans’s patience as he brings one of his phalanges to his teeth, pensively tapping at the sturdy, flat little things.

Another thing comes to mind -- what exactly had swapped? Their LV, their consciousness, sure. But how deep did it go, and how did it happen? Any amount of speculation would only frustrate Red further, and Sans tended to only have eyes for the stars.

So, until they figured out how to get back, they were stuck like this.

Dramatically, Red groans to himself.

“We can’t tell anyone.”

Red actually seems incredulous. “Are you serious? How do you think we’re gonna be able to hide this, exactly? You blushing through my bones like some Baja Blast induced rage and your soft, squishy body with the eyes of a killer. Sure. Dandy. It’ll work out swell.”

Sans stares up at the ceiling, curled in on himself. Red feels just as small, holed up in a glass case not meant for anything more than to walk past in a gallery. Absently, he rubs at his chest. Sans chose to wear a second shirt. He realises it’s because it’s a little cold.

“Motherfucker,” he echoes Sans’s earlier sentiment. “No, it’ll be fine. Just lemme do the talking.”

“So the guy that never shuts up is gonna let me handle the talking?” Sans quips, narrowing his eyes at Red. Red’s got a feeling he’s testing out expressions just to fuck with him. “You rarely let me get a word in normally. What’s anyone gonna think about a loudmouth asshole who’s suddenly shut up?”

“Pfft, well it’s not like you’re so polite,” Red mutters. “God, ok. Yeah. We’re gonna have to figure things out before we bite the bullet and get outta here. Someone’s bound to have heard that.”

“We’re forty feet below ground.”

“And if a tree falls in the forest, yadda yadda.”

Sans rolls his eyes. Red approves. Somehow, he thinks that Sans is gonna fuck up the finely crafted persona he’s built up over the years, but as long as he sticks close by, he’ll be able to correct any misdemeanors. And by misdemeanors he means any foo foo shit that’ll take an axe to his reputation. Shit that’ll get him in trouble back home.

Red inhales a shuddering breath, releasing it in careful puffs. He can feel Sans’s gaze on him and decides to take it elsewhere.

He looks inside the body, pinging a check off his soul. What comes back to him makes him tense, wrapped up in a thick coat of shame. 1 HP. Jeez. He must’ve taken a hit real bad to go down that far. That was careless of him.

Unable to meet Sans’s -- _his_ \-- face, Red preoccupies himself by staggering to his feet. Sans follows suit, somehow stumbling himself, as though he’s not used to such a huge height gap. By which Red means two inches. He grins at Sans, hunching in on himself. Best to keep small and unassuming so he doesn’t earn himself a backhand and accidentally dust this poor bastard, and Red in the process.

“So, we’re gonna have to go home, obviously.”

“Obviously,” Sans echoes, somewhat distracted. Red snaps his fingers to get his attention. “What??”

“What’s up?”

Sans sighs. “Do you always feel like this?”

Red grins suddenly. “You mean badass?”

“No. Like, constantly like you’re gonna crunch. This bone mecha’s all pain.”

Red sneers. “Yeah, well. It’s _my_ bone mecha so you better not fuck it up.” He realises mid-motion that he’s rubbing his chest again. Must be residual ache from Sans’s soul. “Must be rainin’ topside.”

A thoughtful look passes over Sans’s face, which makes Red roll his eyes. He’s never gonna get used to this.

“At least I was right when I told you that you’re a giant pain,” Sans says amiably, a hand reached out to brace Red’s path. When Red swats at him, Sans’s scowl deepens. “Hey, don’t break anything.”

“Maybe next time don’t do anything stupid.”

“Says the guy that dove between me and the machine, but sure. I’m the stupid one,” Sans gambles.

Red stares at him for a good long while, challenging and silent. Then he looks back over to the smouldering pile of rubble and grease, great smears of black powder coating the wall and ceiling around it. There’s no sign of the food, and he’s not sure if that’s a good thing or not.

It’s a detached sensation to see his body meander around without him in it. Like there’s some huge mirror and he can’t see his arms and legs where they’re attached to him. Instead, Sans walks in his usual slouch, which looks completely _wrong._ Sans is going to mess this up.

So Red falls into his familiar gait and elbows him, making Sans emit a scandalised sound. Then Red gestures to him.

“What is that.”

Sans blinks at him.

Red growls again, doubling down. “What’d I say about the stupid faces!?”

Sans barks, “How the fuck am I supposed to act like you when you won’t let me just-” He gestures a little angrily, like he’s still trying to process it all and Red’s not giving him the time to do it. “What? What is it. Just tell me. And if you say one more thing about this fuckin’ face, I’m gonna give it back to you on a skinned deer.”

Red’s brow shoots up. Then Sans appears to realise what he said and covers his mouth. Red is honestly impressed. “Nice job, Sansy.” Then he grimaces, _“…Red.”_

“I’ve never called you Red,” Sans shoots back. “You’re just ‘asshole’ to me.”

Red just winks. “Ok, dove. Save some of the barbs for the boss.”

The flush from Sans’s embarrassment seems to wane a little. “Oh.”

Right. His and Edge’s little flirts. This is probably gonna be hilarious, although it would be if Red wasn’t directly involved. Red exhales an all-weary sigh and rubs at an eye.

“Arright, while we sort shit out, we’re stayin’ at my house.” There’s no argument. “It’ll look less suspicious than me crashin’ at your guys’s place,” he adds.

Sans nods. At least they agree in that sense. Red shambles over in his machismo way and then ends it with a wide spread palm, as if to say “like this”. Sans’s look could wither and dry coral reefs to stone.

“You can’t be serious.”

Red sticks his hands into Sans’s surprisingly cosy hoodie and rocks on his heels. “I’m always serious.”

Sans levels him with another look and Red gestures again. “We’re not leavin’ until you get it right, blue-freeze.”

“God, shut up.”

**Notes for the Chapter:**

> Beetle drew an amazing art for me!! [You can check it out here!!!](https://twitter.com/skerbbie/status/1357554031128846338?s=20) ♥


	2. Chapter 2

**Notes for the Chapter:**

> Sans's POV

Red doesn’t let him rest for a moment, which is on brand for him. Sans wasn’t lying when he said Red’s body was a huge pain. Ever since he had blinked back the stars, every crack and scar introduced themself to him in vivid prickling detail.

Must be raining, indeed.

Red coaches him to swagger around like he normally does. Red’s not all that bigger than Sans is, but Sans still finds those extra couple of inches weird to compensate for. What’s more is that he can’t stop from shooting incredulous looks at his body, it scowling at him like this is  _ his _ fault.

_ (Which ok, it might be. It probably is. Who’s he kidding. It’s all on him that he didn’t pursue Red’s reasoning that yeah, a flip of the switch, throw some food into the rift, and close it again might’ve had a few more caveats other than “things might come out”.) _

He does recognise the way Red’s rubbing at his chest, though. Perhaps he should’ve been a little more forthright about needing another fix before coming down here. Sans isn’t entirely sure how that works, but as it stands, Red tries to hide it. They’ve got their own aches and pains to tune out and it’s no surprise that once outside of their territory, things don’t align anymore.

It’s strange to see his face with red eye lights. Like hell Paps isn’t gonna notice that. Just as much as Edge is gonna slam him against a wall, demanding where his brother is. Red, the fucker, will probably watch the whole affair with a huge grin on his face.

To be honest, Sans doesn’t want to practice. He wants a goddamn nap and maybe a snack. Sans feels disoriented and on top of it all, weirdly listless even as he makes clapbacks to Red’s little barbs. Tiredly, he rubs at his face.

“What’s up,” Red demands, rightly suspicious. “What’d you do?”

“Dude, chill,” Sans sighs. “I’m just tired.”

Red doesn’t approach him. It’s like he makes an effort to keep a distance from him, or that he dislikes himself enough that merely seeing his body from another angle upsets him. Makes him wary. Though they’re not… romantically inclined in any way, Sans can’t help but feel a little betrayed by that.

“Kay,” Red says after a moment of consideration. “Guess we better get on it. You’ll work on your walk as we go.”

Sans grunts in affirmation, only to shut Red up about it. He says nothing about the crouched way Red holds his body, like he’s going to twist himself up into a pretzel if the breeze hits him a certain way.

Sans’s magic has been all out of whack even before this little exchange. He wonders how his body fares as Red leads the way. Red doesn’t volunteer to shortcut them out -- chances are he’s feeling Sans’s body fatigue too. Sans isn’t quite sure what to chalk this body’s eroding energy, but it’s a little worrisome. The city checkpoint is a thirty minute walk from the summit, and they’ve gotta trudge through New Home on top of that.

As far as Sans can see, Red isn’t going to make it far. He’s as tense as a rubber band stretched beyond its limit, and Sans isn’t sure how to prod him into taking a break without Red making a production out of it. Cautiously, he puts his hand out onto his shoulder, feeling a definite change in temperature. He can see why Red bitches at him.

“So,” Red starts quietly, very, very carefully. “When were you gonna tell me.”

Ah. Sans had a feeling about this. Red throws a look over his shoulder, pointedly sending him a glare that could make daggers look friendly. Sans tries to look as non-threatening as he can possibly be. His shrug is as natural in Red’s body as it is in his own.

“…After. When we were done and at home.”  _ Probably, _ he silently adds. Red rolls his eyes at him and carries on. Welp, that’s what he gets for being honest.

But Sans is used to anger, of silent treatments and nagging. He’s not sure what this is. Red’s probably got an arsenal of things to say at his disposal, and Sans isn’t sure what memories Red has access to, if any. He doesn’t necessarily want to find out, so Sans doesn’t try to go snooping.

But it’s like that bit of information doesn’t matter. Little things stand out over time as they walk through the old grey streets. The familiar warmth and buzz of the collar around Red’s throat, humming with Edge’s magic. The dense pile of patched fabric Red calls a jacket. The way his left shoe has a little groove in the sole to accommodate for Red’s rolled ankle. All things he didn’t necessarily look for before, Sans now experiences first hand.

Which is when he belatedly realises that Red might notice a few private things about  _ him. _ How he’s wearing two shirts, his hoodie is worn thin in the pockets and elbows, the ghost of weed high on the collar to keep him calm when he’s out and about. The trick pocket in his jacket to hide his smokes and lighter.

Red stops after a bit, fiddling with the back of his neck like he’s got an itch. Sans wonders if Red finds it strange to have a bare throat after all this time. He’s got half a mind to unbuckle the collar, but the whole thing regarding a collared Sans showing up at Red and Edge’s doorstep would probably make Edge flatline a little.

That and… it feels kind of natural to keep it on.

Red looks at him. Sans stares back, feeling warning prickles creep down his spine. His body looks worn thin, so he does the sensible thing and wordlessly suggests a break by squatting down on the road.

“Dude,” Red objects. “We’re not gettin’ home by nightfall if you decide to sit around.”

“And it’s not really a good idea to test the limits of each other’s magic, is it?” Sans shoots back quietly. “Sit down. I look like shit.”

Red gives him a dour look but doesn’t object. Sans can almost feel the way his body complains when Red crouches down. It’s probably nowhere near the amount of pain Red feels on any of his bad days, but Sans knows his body’s limits.

When Red exhales a shuddering breath, Sans doesn’t miss the way he curls his wrist over his sternum, as though to hide the gesture. “There, happy?”

“I’m pleased as punch, kitten,” Sans drawls, testing one of Red’s pet names. When Red gives him a sharp, challenging look, Sans grins. “I could carry ya.”

“Fuck off-”

“It wouldn’t even put a dent in your rep of being a bad boy. It’d just be Red carryin’ around his sloppy joe. Besides, if you make it collapse, it’s only gonna hurt more in the morning.” Sans grimaces when he realises just how much that reveals about him. That, and how much they can hide from Edge -- which is exactly jack shit.

Red looks at him, assessing what he probably feels is a threat. Sans knows it’s out of habit to question and second guess everything that’s presented to him, but for once he hopes Red would just listen to him. He waits in calm enough silence, ignoring just how  _ empty _ the underground is without any of its former inhabitants.

~

When they resume their walk, it doesn’t take long for Red to slow down. Sans figures there’s about an hour’s worth more exercise left in that body before Red collapses, but thankfully it doesn’t come to that. Red wilts, but he doesn’t make it to the ground. He just trails his fingers along the cool bricks in the wall along the way, as though being close to something to lean on makes him more prepared.

“You realise how stupid this is, right?”

“Yep,” Sans pops, a little relieved when Red finally relents to being propped up. “C’mon, now.”

With a little manhandling that takes less force than Red puts a show on for, Sans stoops so Red can half-assedly hang over his shoulders and hook his wrists under his skull. Sans shrugs him up and secures his hold by locking his hands under Red’s ass, raising to his feet, and getting Red to slouch forward.

“Comfy?” he says politely.

“Fuck you.” There’s no heat behind the sentiment, but Red touches the collar at Sans’s throat. It hums to life, sending a subtle shock and enquiry through the brief connection. Sans has a feeling it’s more for reassurance than to ping their location to Edge, but he doesn’t want to touch that bruise. “Giddy-up.”

“Yeah, yeah.”

For all his bitching and moaning, none of it actually carries on as Sans walks. Red’s body boasts a surprising amount of stamina, and apparently not only in the bedroom. Sans makes his way out the only way he knows, winding around tight streets and alleyways all the way through the capital, to the castle, then through the throne room and beyond. He doesn’t hear any complaints from Red, only the soft gusts of his breath in measured, careful sighs.

Sky breaks into a wash of filtered blues and purples. Beyond all the heavy grey clouds across the horizon lies the first whispers of sunset, as though scraped with a palette knife. It’s mid-autumn, which makes the summit colder; Red likely feels the brunt of it since he flinches right into Sans’s spine. It doesn’t help that the scent of rain is in the air, the ground wet under his feet.

“Want your jacket?” Sans offers quietly. Red’s hold on him tightens on him.

“God, this is either really bad, or your pain tolerance is through the roof,” Red grunts sourly. “You’re a fuckin’ shithead, you know that?”

“I had a hunch. Anyway, you wannit?”

“Gimme,” Red says shamelessly.

So Sans does. Carefully, he stoops down so Red can dismount him like some kind of bony pack mule and then gets up to shrug off Red’s jacket. Red tends to run a little warm anyway, but Sans feels smaller  _ (exposed) _ without the extra bulk. Red considers him for a moment when Sans holds out the jacket for him, then takes it and gratefully throws it over his shoulders, slinking his arms through the sleeves.

It leaves Red’s body in one of his heavy metal tees, the one with the skeleton with hair throwing the horns while spiky text sweeps over the front in an illegible scrawl. In all honesty, Sans likes this one. He thinks he has its twin somewhere at home.

“Better?”

Red’s normally toothsome grin is a little undercut by the absence of shark teeth. “If you keep checkin’ up on me, I’m gonna develop a complex, Sansy.”

Then he grimaces like he just got stung by a wasp. The voice swap is a little more than jarring.

Sans just waves it aside like it didn’t happen. “Before we go any further, we got some ground rules to lay out,” he says.

“Right,” Red replies, mock deadpan. “Of course we do.”

“Firstly, in case we do have to split up-” Sans begins. “Me n’ Paps don’t have the same, uh… fraternal bond you and Edge share.”

Red’s look gets suddenly, incredibly wicked. Sans never wants to see that expression on his own face ever again.

“Oh. Well, then… good luck with the boss.”

Ignoring it to the point of blindness, Sans barrells on, though he can feel the heat of what’s implied creep up his throat.

“You gotta stop hunching like that. No one’s gonna stab you.”

Red scoffs derisively. “Easy for you to say. I took a hit. An acorn could finish me off now.”

Sans shrugs. Then he pings a Check off the soul housed in the body he’s in. 4 HP. Not bad, considering Red maxes out at 5. No wonder he’s able to hike across New Home and not even get winded.

He considers that for a moment. Amused, Sans relishes the scandalised look Red gives him when he sends a Check his way, reading what comes back clear as a bell; “1 HP. You’re fine.”

“Wh-” As Red checks for himself, Sans feels some kind of relief. Not smugness, no, that’s Red’s department, but Sans feels at ease knowing Red didn’t take a huge hit protecting his fragile ass. “Oh. Alright. Uh, good. Now that it’s off the table, I’ll just step around sharp corners and anything not packed in spray foam and pillows.”

Sans rolls his eyes. He has a feeling he’s got the better end of the stick when it comes to the swap.

“You gotta stop stumbling around like a drunken toddler, though,” Red says almost indignantly.

“Yeah. That’s gonna be a hard habit to kick.”

“You’re givin’ away too many tells when you’re in there,” Red adds, pointing. Sans realises how wrong it feels to have Red squinting, grimacing, and roughly cussing in his body.

“I’m not thrilled about it either, dude,” Sans shoots back. “We’re obviously gonna need time to settle.”

“No offense, but I’m not settlin’ in this eggshell you call a home.”

Sans sighs. Yeah, whatever. He just lets that slide. There’s no use in making things a bumpier road than they ought to be, so he just rests.

“Alright,” he says after a moment. “If we hustle, we can make the 6:40 to Glenrow. Get up.”

Red makes a sour noise of opposition, but he obeys Sans’s request sure enough. When Sans assumes the position to resume their earlier transportation woes, Red clicks his tongue, but climbs back onto Sans’s back.


	3. Chapter 3

**Notes for the Chapter:**

> Red's POV

It’s hard to concentrate.

Red doesn’t remember soul pain being this bad. He’s of two minds to crack his fist over his skull about not catching it sooner, for not being attuned to how Sans felt over the past week or more, or to just stay quiet. There’s a feral need to blame himself, because obviously Sans is set on self-destruction even when he’s healing and there’s nothing he can do about it, but hey. Idle minds and whatnot.

Instead, he tries to relax, curled up against his body as Sans pilots it down the slope. It’s weird to not shove it away, like being close to that thing and Sans internally freaking out about the swap is weighing heavily on his psyche. Red doesn’t deserve comfort, but that’s  _ Sans _ in there. He’s trying not to bite and lash out, so being an asshole is next on the docket.

He should say something. It’s Red’s fault they’re here -- a lapse in judgement that he could throw a sack of rations into the portal and close it before anyone knew about it. Just tuck it in behind the library and no one would know until it was discovered by some kids. And then the machine started to smoke.

Red huffs a sigh against the warm leather around Sans’s throat. His own neck prickles and itches, his mind rising through heavy nerves and waning again when he realises that it’s not his own panic, but  _ Sans’s. _ His body is scared for whatever’s inside of it, a foreign entity not meant to share the same space Sans calls home.

Eggshell, indeed.

There’s probably a deeper reason as to why he’s feeling Sans’s soul pain. Red can feel it intimately, encased around him like cupped hands pressed tight at the seams. He figures that a synchronicity bounced them into each other’s body. And they ended up, miraculously, safe in the closest soul available.

Which probably meant that the force of the energy knocked Red’s essence forward and out, forcing Sans to react in kind.

Or at least… he thinks so. It could be bullshit, his mind grasping for straws to make sense of it all. It’s getting harder to think beyond the static of pain. How the fuck Sans felt this was appropriate levels of agony to blank out is beyond him. Or maybe, the switch-up did something to jar Sans’s soul. Aggravate the cracks. Who the fuck knows.

“You’re suspiciously quiet. You doin’ ok?” he hears his voice say. Blearily, Red looks over his shoulder and down the rest of the hill to the transit loop and their little shelters. A scattering of people linger around as a couple of buses idle until departure.

“Your soul hurts,” Red mutters sourly. “If it was this bad, you should’ve said something.”

“It wasn’t that bad,” Sans replies, though if he’s faking honesty, it’s shrouded by the fact that he’s speaking with Red’s voice. “It didn’t feel bad this morning. Maybe whatever happened messed with it.”

Red hums in rough affirmation, keeping his eyes on the road as Sans approaches the main walkway. He can feel the body under him tense up around the group of strangers, as well as a prickling unease that wells up inside of him. Inside of Sans.

“You got fare?” Sans asks over his shoulder.

“Should be some bitch change in slot 8,” Red indicates, not nuzzling close but it’s a close approximation. He figures their inventory access is swapped too. Curious to see what Sans has in his innermost pockets, Red mentally grabs at the first thing presented to him, only for his reach to be blocked.

He tries again. The body under him tenses a little more, then Sans lets go of one of his legs to fish through Red’s pocket.

Red’s mouth goes dry. He tries again, a glanced blow at his fingertips like he’s trying to reach through the glass window of a vending machine.

“Uh…” Sans mumbles very quietly. “You only got maybe a buck and change in your pocket.” He stays silent for a moment. “Can you, uh. Check my inventory?”

Red growls low in his throat. “I tried. Your inventory’s butt is so tight it squeaks.”

Sans snickers but he turns his head a little more. “Be serious or we’re gonna miss the bus.”

“I’m bein’ serious,” Red grouses. He tries again, this time aiming for his own keepsakes. It buzzes like a static charge, but no familiar wisp of warm magic presents itself to his fingers. “I can’t get into my inventory. I can’t get into yours.  _ You _ try.”

There’s a pause as Sans makes another attempt. The humans around send them odd looks as they board the nearby bus, but Sans doesn’t pay them any mind.

“C’n you… y’know,” Sans falters, swinging his arm behind him to hold Red up again. “Get us outta here?”

Red doesn’t say anything for a moment. He considers it, wonders if it’s his magic or Sans’s in this body, or if they’re just two consciousnesses trapped. His breath shudders out when he tries for the barest tug at his reserves, red and as hot as a welt. He can feel the molten-ice casing of Sans’s soul close in on him like a vice and he gasps.

“Nope. No shortcuts today, honey. You’re too fucked up.”

He can hear Sans nervously swallow with a dry click. “Thanks. I always wanted to hear that comin’ from you.”

Red just sighs and slumps over Sans’s shoulder, leaving the barb alone. “Just try to mooch a ride from the driver. If hobos do it every day then we should be able to score one.”

If Sans was incapacitated instead, Red would just glare at the driver and move towards the back of the bus without so much as an attempt, but Sans has hang-ups about not being murdered or raising a ruckus. Which is fair, as Red’s grown accustomed to his body and he’d like it back in more or less one piece.

To his credit, Sans approaches the kneeled bus like he owns it. Red keeps his eyes closed to feign rest, though honestly it’s because he feels as shitty as he did when he got the crap kicked out of him shortly before leaving home. Oddly enough, Sans moves forward, then Red hears the slide of the few coins he has clink into the fare machine.

“It’s all we got. We just wanna get home,” Sans says with minimal apology. The driver must just wave to him, as Sans adds, “Thanks,” and keeps moving down the length of the bus.

With nothing in their pockets and no food or way to communicate with their brothers, they sit at the back of the bus where it’s empty. Red curls up against the window, the waft of the heater at his feet downright heavenly after the bone-deep chill of the mountain. Sans sits next to him, pressed against his side like a protective dog.

“Better now that you’re not moving?”

Red grunts quietly in the affirmative, his skull rattling gently against the window as the bus starts to move. Sans clicks his tongue and pulls him away from the jarring bumps, holding Red to his side. It’s not uncomfortable.

“You’re not that fragile, kitten,” Red mutters quietly. “It was distracting.”

“It was  _ annoying,” _ Sans corrects, his voice a little strained as though he’s trying not to sound as irritated as he feels. Joke’s on him; Red’s voice has a way of sounding irritated 24/7. “Just relax.”

Red just grunts again, seemingly content to lay against Sans to soak up his warmth. He doesn’t loop Sans’s arm around his own and sticks his hands into his pockets, no, but Sans doesn’t say anything when they wind up like that over the slow drive of forty minutes.

He’s nearly drowsing on the bus, which is a new experience for Red. Typically he’s alert and too wound up to relax, as everyone that passes by gives him a flash of suspicious defensiveness like he’s gonna jump them. Quietly, Red glances to Sans to see how he’s doing.

If the innermost workings of his mind are anything to go by, Sans is taking this rather well. It might even be something of a buffer, a breath of fresh air to not be so wound tight in the glass case of pain he calls a body. Of course, Red would never really (truly) rub it in Sans’s face, but the less Red moves, the better he feels. Now he knows why Sans isn’t really up to tasks that involve moving around for hours at a time. Or hell, minutes.

When the bus follows the curve of the hill down towards town, Sans straightens. He’s been quiet the entire time. His eyes are distant as he reads the roads from the central window. Red can feel him tense, which is the only reason Red realises that he’s been dozing.

“We there?” Red rasps quietly. He hopes it’s mostly from exhaustion and not because his magic’s so loose and thin that it’s a little worrisome.

“Close to,” Sans murmurs. He doesn’t say anything more, but he gives Red’s arm a squeeze from where it’s crooked into his elbow. “I was thinking.”

Red snickers. “What you got, babydoll? I was wondering what the smoke was.”

Sans levels him with a withering look, which only makes Red preen despite how crazy it is to be on the receiving end of his own fucking expressions. Sans is making him too easy to read. It’s becoming a problem.

“Once we’re off the bus,” Sans says, ever-ominous. Red just heaves one of Sans’s disparaging sighs for extra effect. “Can it.”

“Can it,” Red snickers again. “You tasting all my vocabulary or just the ones you think will fly with the boss?”

Sans’s face doesn’t redden like Red’s face should. It blears into a gentle pale blue, deepening slightly in colour. Like hell Edge isn’t gonna notice that right away. Sans doesn’t verbally tip his hand like he normally would, so Red does him a favour, trying the phrase for himself.

“Shut up, Red.”

That gets a crooked, half-repressed quirk of Red’s own mouth to tick up. As though to save face, Sans just grunts and turns his head away.

As though he’s been paying attention for the past couple of years in Red’s company, Sans waits until the bus is empty to get up. Red shoots him a pained look when he rises, having been lulled by both a warm body and the flush of the heater at his feet. He doesn’t want to move.

Sans apparently takes pity on him, but not before giving Red a look that plainly says so. Red bites back a grumble as Sans leans over him to help him stand, Red’s legs shaky and loose like he doesn’t have the strength to stand.

It’s annoying. His body isn’t like this. He’d say fuck Sans for making his body this way, but that’s neither fair or warranted. Sure, Sans waited, or whatever, but Red can’t blame him all the way to Sunday for having a shitty sense of self-preservation. Or he can, and be the world’s biggest asshole.

But he’s tired.

He resigns himself to it, but he doesn’t give Sans any leeway. He curls into his body heat like a frozen frog’s last days in autumn, trying to hold on so his joints don’t ache as much as they do. Sans just gathers him up, easy as can be. Red already misses the strength he normally possesses.

True to his word, Sans waits until they’re well away from anyone who could overhear them. In fact, Sans nearly shoulders Red, holding his rear under his arm and supporting him by pressing Red to his chest. Red doesn’t miss the subtle shiver; chances are Sans feels it as much as he does. He’s as cold as an ice cube.

“What’d you have in mind, sweetheart?” The casual sleepiness with which Red speaks makes him pause, because Sans’s voice oddly enough sounds good like this.

Sans gives a slight cough as though to clear his throat. “One, is to uh. Take care of me until Edge gets back.”

Right. Edge. Red had forgotten about his little brother’s appointments in the entire upheaval. He already knows of Sans’s downright refusal to involve Paps in any kind of healing interaction, which is a shame, Red finds. He could use some lurid fantasies right now to chase off the chill.

That leaves the good old fashioned way. Instead of calling out the obvious, Red waits until he can feel Sans shift him, awkwardly trying to slip in the words without speaking. Red’s an asshole. He can wait.

“What do you mean?” Red says with mock obliviousness. He can hear the click of his jaw go as Sans swallows again. Damn, he’s too easy. “Oh. You balk at fuckin’ me but it’s weird to bring up the actual ‘fuck myself with my actual face and voice’?”

Sans’s head moves like he’s looking around to see if anyone was near enough to hear that. Red wonders if he’s as blue as the sea now, embarrassed. Hilarious.

“Might be fun,” Red says, sprawled over Sans’s shoulders as bonelessly as possible.

“Awkward, you mean,” Sans mutters.

Red just clicks his tongue. “Either that or you try to heal your own damn soul with whatever’s goin’ on. And if not, hey, you can always do me from behind.”

Sans stays quiet. Another blistering tell. Red laughs quietly, the sound deepening the ache in his chest.

“Whatever you decide to do, if you don’t, I’m gonna try to take care of it. Simple as that.”

Sans has the nerve to sound scandalised with his own goddamn voice. “Fuck, don’t you dare.”

Red’s grin gets wicked. It’s a shame Sans can’t see it, buried against the collar. “Maybe it’ll be fun,” he teases relentlessly into the crook of Sans’s neck. “Maybe I’ll park myself in front of a wall-length mirror so I can see your face do all kinds of pretty things when I wreck you.”

Sans goes tense and quiet. He’s probably making very sure not to drop Red on his ass, which is to say his own. Red would. In fact, if Sans doesn’t do anything about this goddamn soul pain, Red might just rub one out for the hell of it. It’ll be nice to see how loud he can hear Sans’s voice go, bring it up to a breaking point, and-

“We’ll, uh. See.” Well damn. Sans is just full of surprises today.


	4. Chapter 4

**Notes for the Chapter:**

> Sans's POV.

The first thing they do when they get to Red’s place is quietly look for the spare key. Their magic is all out of whack, but neither of their bodies trigger the wards Edge and Red have set up around their home. For a startling moment, Sans thinks he hears a wail from the neighbourhood kid, but Red just snorts into his neck.

“Fuckin’ cat.”

Right. Sans inhales a shuddering breath as he locates the spare key under one of the sliding panels concealed in the siding trim. It buzzes in his hand like it’s not meant for him. Red decides to take that as a hint to crawl off his back.

The key slides in easily enough, turning in the channel with a gratingness that makes Sans’s nerves kick into overdrive. He’s expecting an explosion. The fact that Red steps to the side a little makes him brace for it, but when Sans opens the door, nothing sinister happens. He spots the mottled face of Fang at his feet, all squidged together like some poor misshapen pug/persian mix. When he opens his mouth, he squawks at Sans.

“I’m surprised he didn’t launch himself atcha,” Red says helpfully from Sans’s back. One of Fang’s tattered ears flicks outwards and he gives another raptor-like ‘mrah’. “Yeah, yeah, fuck you too.”

Sans oddly enough doesn’t feel at ease, even though Red apparently does. The body he’s in has been slowly curling inward, like he feels the residual need from his  _ own, _ pressed against himself so tight over the past few hours. Red slowly approaches the door, takes an appraising look around, then apparently deems it safe enough to enter.

So why can’t Sans shake the weird feeling?

“Wanna try?” Red asks from across the living room, already out of Sans’s sneakers. He didn’t remove the jacket, which is probably good because Sans’s body needs the extra warmth. “I’m into it.”

“You’re into a lot of things if it’s debased enough.”

Red gives a nonchalant shrug. It looks weird, all of Red’s expressions and gestures perfectly mirrored onto Sans’s body like a paper doll. “Eh, it’d just be easier if you do all the work at this point.”

Sans just rubs his face, mentally weary from a long day of hiking up the mountain, getting fucked by reality, having to deal with this freaky friday bullshit, and then having to trek back down while carrying his own self. The fact that Red’s body can do all that and still remain upright is downright awe-inspiring. He’d never tell the smug bastard, but colour Sans a little impressed.

“Dinner?”

Red’s grin gets wicked. “Ooh.”

“Fuck, I don’t-” Sans failed that test. He rubs at his face a little more. He’s not sure why this is flustering him as much as it is. It’s like his emotions are sliding out of place and he can’t stop them from creeping off the ledge. “Whatever, yeah. Some food before we try anything.”

Red rocks on his heels once before realising that it’s probably a bad idea, if his grimace is anything to go by. Sans just rolls his eyes and him and kicks off Red’s admittedly too-comfortable shoes and nudges them beside his own, then meanders over to the kitchen.

“If you say anything about how I make your body waddle, I’m gonna cuff ya,” he calls over his shoulder. Red only scoffs and flops down onto the couch with a groan like an old man. “My body be damned,” he adds under his breath.

Sans pauses when he’s in the middle of the kitchen, like the portal to a new room suddenly blanks the reason why he’d entered it from his mind. He lingers, malaise and  _ something else _ inching over his psyche like a wave of crawling insects. It feels too unfamiliar, adrenaline threatening to shoot out like he’s skirting too close to oncoming traffic.

Maybe he’s just anxious. If he dwells on it too much, Sans is going to go crazy. Let alone if it’s something Red has to deal with too, and he seems to be taking it…

…well, taking it all in relative stride, all things considered.

Sans peeks out of the kitchen when he hears soft noises from the television. Then he spots some kind of deeply concentrated look on his face like Red’s trying something, and he catches the moment it doesn’t work. Red’s breath catches slightly, then he looks over and gives Sans the finger, his normally sharp grin off-kilter on Sans’s face.

“I’m not gonna kick it while you’re dishin’ out sammies, Sansy,” Red drawls as he slides down the back of the couch. It spreads his femurs in a vulgar way, parting Red’s jacket and inching up Sans’s hoodie and shirts well past what’s acceptable. His hand lays on his thigh, drawing Sans’s focus, as though it’d just be like Red to tease him like this. To push boundaries Sans didn’t know had to be crossed.

And well, if his body has anything to say on the matter, that might just be the hard line he  _ can’t _ cross. But then again, Sans thought that when Red offered to pop his sabbatical cherry.

“Or maybe I will,” Red continues, not even bothering to look up from some police procedural. “Who knows.”

Sans just gives a quiet sigh and turns back into the kitchen in search of food. Lucky for them, the fridge is stocked with buns, fresh lettuce, and some deli meats, so he sets about assembling some sloppily made sandwiches. Fang reappears to slink around his legs, so Sans conveniently drops a scrap or two of ham and pats his back.

When he brings out the plate, he parks Red’s bony ass onto the couch next to him and puts the plate between them, taking one of the halves for himself. Red coos something ridiculous, but it doesn’t have its usual sting.

Or maybe Sans is just tuning him out better, distracted by Fang bumping his big doofy head against his leg to beg for more ham.

That is, until Red snaps his fingers at him. Sans glances over, mid-chew, and his expression is all wrong. Red’s making his face do the concerned thing, scrunching up in suspicion. Either that or he’s trying to get a rise out of him.

Sans glowers at him. “What?”

Red doesn’t ask him if he’s ok. He sits there for a moment, reading him, and Sans can see the cogs turning. Then, like he didn’t do the weirdest thing in the world, Red eases back on the couch, sinking bonelessly into the cushions.

“Nothin’. Just thought I saw somethin’,” Red says casually, like he’s not clearly hiding something.

Sans decides to ignore it; he’s too tired to argue, so he settles on being irritating. It’s only fair. “It’s your face,” he says between an ill-torn mouthful of sandwich. “You’re gonna be seein’ a lot of it until we figure things out.”

“Likewise,” Red snerks. “Afterwards, especially. You try gettin’ my phone out?”

“Is that what you were trying to do?” Sans says. When Red rolls his head on his shoulder, sending him a depreciative look, Sans sighs again, world-weary. “I’m starting to hate myself again.”

Red nudges his leg against Sans’s, a mock tease. “Welp, then I suppose it’ll be up to me to show you how to self-love again.”

“Gross,” Sans automatically snarks.

Red bites into the sandwich, appraising it like it’s the best thing he’s eaten in days. “Maybe,” he says. “You got hang-ups now of all times?”

Sans does. His brain’s been speed-dialling past the way they usually settle his soul pain. To see his own face twisted in ecstasy, his own voice breaking on moans that Red normally gives him… Suffice to say, it’s a little weird, even for them.

“Maybe you c’n take me from behind,” Red offers casually after another bite. “Doesn’t have to be tab A into slot B-” Sans feels his face heat but he doesn’t look up. “Just reach around and do what you normally do to yourself.”

Yeah, Sans is definitely blushing. He could enact all the dirty little secrets he keeps in his innermost fantasies when he’s alone, doing exactly what he’s always wanted Red to do to him. But that’d be tipping his hand too much, tossing everything vulnerable out in the open. Sans just stays quiet, his mind slowly processing it.

“Nope, I can’t get into your inventory,” Sans relents, going for broke and pretending that entire conversation didn’t happen. He pauses to try, just in case, and it doesn’t work. It’s merely a peek into what Red’s got in his pockets, but he can’t get a hold of anything.

Red just snickers. “I’d love to beg in your voice.”

The heat from Sans’s face doesn’t abate, but he taps against his skull with a finger as though to test his acoustic meatus. “Weird, I’ve gone deaf.”

“You want me to repeat what I said, huh? Over and over?”

“God, never.”

“C’mon,  _ Red,” _ Red devilishly simpers, leaning ever so slightly into Sans’s personal space. “You c’n shut me up yourself however you like.”

After a moment of tense silence, Red mercifully eases back onto the couch as Sans’s mind races with beautiful, pornographic imagery. None of it would help his own soul, but it’d sure make the going easier if Red had his mouth full. He swallows as discreetly as possible, feeling both ease and disappointment when Red relents.

“Fine, spoilsport.”

Waiting until he’s done eating, Sans nudges Red’s leg with his knee. “Did you think about looking at ‘em yet?”

It’s a simple question, but loaded.  _ Did you want a peek at my soul _ has a lot more meaning beyond touch, when neither of them know what happened to make the switch. Sans remains still when Red regards him, then sits up.

“You want to?”

Sans just spreads his hands. “I’m just thinkin’ we should… probably… I dunno.” Check it for new cracks, check to make sure it’s not bleeding. Maybe see if there’s an inkling of Red in there, since he feels gripped around his very being by Red’s soul. It’s probably the same for him… right?

Would his soul sustain him? He’s gotta admit, he’s not fond of the thought passing through his head that a wrong move can make his body a liability to Red now as well as himself.

Sans finds himself saying, “We’ll check yours too. Just in case.”

The lecherous smirk at Red’s --  _ Sans’s _ \-- teeth is all but gone, but he mercifully tames it. Probably just for Sans’s benefit. At least, he hopes so. Red leans up and squints an eye, a hand on his chest. A small glow starts behind his cupped fingers as he draws it out, and it’s  _ wrong. _

The resonance is all off-key, chaotic and wild where Sans is, well, not as much. Beyond the old, thin pale crack, the magic inside twists around like a storm, and at its eye lies something outlier and familiar.

It’s Red.

Trapped in the tempest, there’s a clear disturbance that Sans can see. He  _ knows _ it’s Red, but it’s difficult to put words to. It’s like knowing a glass of water is still moving after stirring it, the tumultuous resonance apparent all at once. But at the centre of this metaphor is a small jellyfish, trapped in the glass. If they attempt to extract it… Well, without a proper conduit, the jellyfish would…

He doesn’t want to think about what would happen to Red if they’re hasty.

He must not be able to school his expression as much as he hopes. Red surveys the dim organ, though it’s not as dim as all the other times Sans’s soul needed healing. If anything, Red’s interference is likely to blame for the aches. Outwardly, it looks the same as it always does.

“Alright,” Red says quietly. “That’s… probably why.”

Sans can’t get out the words.  _ Probably why _ like Red knows what the fuck happened. Like he’s not concerned at all. Out of habit, Sans rubs over his sternum, catching the barb of a scar on Red’s wrist. Red gives him a look, but since he’s spoiled, he gestures for Sans to lean forward.

“Hold on, lemme see mine.”

It’s a quiet request. Sans holds still, tension thrust up his spine as Red draws out his soul. It’s different, yet not quite right. It doesn’t seem like something Sans should be able to look at, the webbing of mismatched cracks unnaturally calm on its surface with a spike of fever just under it all. A current drags in the fluid within, swirling around his essence in Red’s soul.

It feels almost vulgar, like he isn’t meant to stare. Hesitantly, Sans jerks his gaze away, to the less-intimate display of his own soul in Red’s other hand.

No, it still feels intrusive. Mixed up in each other’s cores, occupying one another’s bodies. He wonders if they’ll be able to swap back again.

“Well,” Red murmurs quietly. “It sure is… somethin’.”

Sans swallows thickly, keeping his eyes to himself, very aware when Red presses his soul back into his body.

“At any rate, it doesn’t look like you’re dyin’, which is a nice change,” Red adds thoughtfully. He gives Sans’s chest a firm pat once he puts his soul away, like it wasn’t the most intimate gesture they’ve had so far. Sans keeps quiet. “N’me either, in case you’re wondering.”

“Kay,” Sans dryly agrees. His mind wants to pile over so many what-ifs and hypotheticals until his brains rot, but he disconnects from it until later. “Guess we don’t need to-” He gestures vaguely. “Well.”

Red’s grin returns to its usual wickedness. Sans can tell that he’s keeping a little out of reach, like he doesn’t quite trust himself, let alone  _ like _ himself, but… maybe he’s fighting that urge.

And to be honest, Sans could use a distraction from all the mixed signals he’s getting.

“Well,” Red says, tasting the words with Sans’s voice, all soft and dozy like after a good nap or a good fuck. “I wouldn’t say no if you’re curious.”

Funnily enough, that’s Sans’s whole life right there. Colour him curious.


End file.
